The growing market of ideas that require personal power ranges from electronic and telecommunication equipment (e.g., cellular telephones and laptop computers) to small, mobile reconnaissance robots that can safety explore potentially hazardous environments. Many of these lightweight devices demand tens of Watts of power for durations on the order of tens of hours, thereby driving power source considerations toward those power sources with the highest energy density.
The energy density of burning hydrocarbon fuels is difficult to surpass when an oxidizer stream is plentiful, as with combustion in ambient air. Assuming no energy cost for the oxidizer, for example, typical hydrocarbon fuels can provide a power density of 45 MJ/kg, while a modern rechargeable battery can only manage a mere 0.5 MJ/kg. Even fuel cells, while highly touted for their efficiency and simplicity, only provide power densities comparable to batteries, i.e., 0.7 MK/kg. Perhaps more importantly, the energy per unit volume of electrochemical devices is quite low because they rely on surface reactions, while combustion is a volumetric energy release process. Consequently, if the ultimate goal of a power device is propulsive or heating, direct combustion will have clear advantages. Even when electrical power is desired, where the combustor dimensions are often a small fraction of the volume occupied by the conversion hardware, if high power density is needed, combustion technology tends to still hold clear advantages.
Because internal combustion has the potential to simultaneously provide high power density and high energy density, many researchers have attempted to explore it as a method for power generation on a miniature scale. Examples of such exploration include, a micro-gas turbine with a combustor volume of 0.04 cubic centimeters (see Waitz et al., 120 Jnl. Fluids Engr., 109-117 (1998)), a mini (0.078 cc displacement) and a micro (0.0017 cc displacement) rotary engine (see Fu et al., 99F023 Combustion Inst., Western States Sect., Fall Mtg. (1999)), a microrocket with a 0.1 cubic centimeter combustion chamber (see Lindsay et al., IEEE Cat. No. 01CH37090, 606-610 (2001)), and a micro Swiss roll burner (see Sitzki et al., 3rd A-P Conf. Combustion (2001)). Although these devices have demonstrated the plausibility of internal combustion as a personal power source, they are not able to perform at efficiencies that make them competitive with the best available batteries.
A major challenge for all miniature combustion concepts is the increasing surface-to-volume ratio (S/V) with decreasing combustor size (since this ratio scales as the inverse of the combustor length scale), where the volume is the combustor volume and the surface is the area of the wall surface that bounds the combustor volume. Because wall temperatures are generally kept fairly low due to material considerations, a high S/V ratio results in high heat transfer losses and, thus, usually produces flame quenching, particularly for premixed flames. Attempting to overcome such problems, researchers have turned to quench-resistant fuels such as hydrogen gas, combustion chambers with catalytic surfaces, or high-preheat concepts such as the Swiss roll burner (Sitzki et al.).
Thus, it is desirable to provide an efficient miniature combustor capable of burning typical hydrocarbon fuels while avoiding quenching.